Seasons and Solstices
Why are there seasons? For many years, people pondered this same question, and
it took humankind until the 1500s to finally figure it all out. The solution, it
turns out, is as simple as the Earth orbiting the Sun. Almost every person born
since the Civil War has known this since he (or she) was a child, but still,
just because the Earth goes 'round the sun, this does not imply that seasons
should necessarily occur; there's something a little more complex than just the
orbit of the Earth involved. This section of the tutorial shall examine how it
all works, and why, exactly, there are seasons.
A few terms you ought to knowEquinox
- A time at which the days and nights are the same length around the world.
- Occurs around March 21 and September 21 (but not necessarily on
- Occurs when the Sun is directly over the equator.
- Is either vernal (in the spring) or autumnal (in the fall).
- A time at which either day or night is the longest it will be during the
- Occurs around June 21 and December 20 (but not necessarily on those
- Goes simply by winter or summer solstice.
- Occurs when the sun is directly above 23.5 N latitude (Summer Solstice) or
23.5 S latitude (Winter Solstice).
- Will allow one pole to have 24 hours of daylight, while the other pole has
a 24 hour night.
The Axis of the EarthThe Earth spins on its axis, an invisible
line through the center of the Earth. The northernmost point of this axis is the
North Pole. The southernmost point, therefore, is the South Pole. The Equator is
an invisible line that encircles the widest point of the Earth, and is
equidistant from either pole at every point; that is to say, the Equator
is the same distance away from each pole at every point along it.
rotates along the plane of the equator, meaning that the Earth spins in a circle
represented by the equator, or any latitude line, for that matter. (It wobbles a
little, actually, but it's such a small wobble that it really doesn't matter
much unless you're thinking in terms of 26,000 years at a time.) If you stood
out in space so that you could look down over the North Pole, you would notice
that the Earth spins counterclockwise, which makes sense if you consider that
the sun rises in the East and sets in the West.
North Polar and Equatorial views of Earth and Solar BeamsThe
interesting thing about all this is that, even though the Earth rotates on the
plane of the equator, the Sun doesn't always follow the equatorial path. In
fact, the plane that the Sun appears to follow as Earth rotates is dependent
upon the time of the year. It's a little tough to grasp at first, but it's this
tilt that gives us the seasons.
Earth's axis tilt and equatorial plane compared with its plane of
orbitEven atmospheric scientists have trouble with this concept at
first, so don't be discouraged. If it still isn't that clear, click on the
picture, and you'll go to a place with instant, Java-friendly animations, and a
more extensive explanation.